428 PHYSIOLOGICAL SERIES. 



The parieto-occipital sulcus is now a simple sulcus with 

 external (dorsal) extent almost as great as its internal 

 portion. It is separated from the intraparietal and Simian 

 sulci by a wide gyms on the left hemisphere. On tin- 

 right hemisphere, the Simian sulcus curves forward so as 

 almost to reach the parieto-occipital sulcus. 



The calcarine sulcus presents upon the left hemisphere 

 the usual bifid form common to most Apes, whereas upon 

 the left side it is plain and undivided, as in most Sem/io- 

 pitJieci. The occipital operculum is smaller even than it is 

 in the Langur-family, and the inferior occipital sulcu- is 

 reduced to a mere vestige, which is triradiate on the left 

 hemisphere. 



The lateral occipital sulcus, which is well developed in 

 Semnopithecus in comparison with other Apes, is even 

 better developed in this brain. It is a deeply-incised 

 Y-shaped sulcus, the two limbs of the Y embracing the 

 caudal end of the calcarine sulcus. 



The sulci rectus and arcuatus are concurrent, as they are 

 in many of the lower Apes. 



But the most characteristic feature of the brain in the 

 Gibbons is the condition of the insular region, which 

 distinguishes this brain from that of all mammals except the 

 Anthropoid Apes. The fronto-orbital sulcus is especially 

 well-developed (see the left hemisphere), and now con- 

 stitutes the anterior limit of the exposed part of the insula, 

 the dorsal boundary of which has been mapped out by the 

 superior limiting (Marchand's " opercular ") sulcus, which 

 has emerged from the Sylvian fissure to a much greater 

 extent than in any of the lowlier Apes. 0. 0. 1338 Q a. 



D.645. The brain of a Gibbon (Hylobatea lar). The left 

 hemisphere has been separated. 



This is a smaller and somewhat simpler brain than the 

 above. 



The calcarine sulcus is unbranched on both hemisph< 

 The ramus postcentralis superior of the intraparietal (which 

 is very well developed and united to the main sulcus in the 

 last specimen) is a mere disconnected depression. The 

 two-limbed sulcus formed by the union of the superior 



